The absorption of Radio Frequency (RF) radiation is a desirable solution to many Electromagnetic Interference (EMI) applications, with the alternative being the reflectance of the radiation. RF absorbing materials are well known, but most of these materials are optically opaque. RF absorbing materials do not exist that are suitable for use as a high quality optical window.
Jaumann absorbers are an established class of broadband RF absorbers composed of thin electrically conductive layers separated by spacers having a low dielectric constant. The conductivity of the thin layers increases (sometimes exponentially) with position away from the surface. The RF radiation is therefore allowed to penetrate into the absorber before being absorbed. Jaumann absorbers are sensitive to the angle of incidence, but the absorption bandwidth of a Jaumann absorber can be increased by the addition of additional conductive sheets with spacers. However, Jaumann absorbers are fabricated from optically opaque materials.
There are Electromagnetic Interference (EMI) applications in which an optical window with RF absorbing properties is desirable. One example is an optical viewing port in a radar test chamber where the internal walls must be absorbing.